1. Field of the Invention
The present invention relates to a lithographic apparatus, a controller to control a position quantity of a first and a second movable element, and a method to control a position quantity of the first and second movable elements.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus comprises a reticle stage to hold the reticle and a wafer stage to hold a wafer. A reticle stage positioning system is provided to position the reticle stage and a wafer stage positioning system is provided to position the wafer stage. The positioning systems may comprise e.g., control loops arranged for a positioning of the respective stages. As the pattern provided by the reticle is to be projected onto the wafer with a high accuracy, an accurate positioning of the reticle stage as well as the wafer stage is important. Furthermore, an accurate tracking of a position of the reticle stage and a position of the wafer stage (thus a low relative position error) is desired. If both stages show a position error, an effect thereof may be substantially less in a situation where an actual position of the reticle “follows” an actual position of the wafer stage, as compared to a situation where position errors of the stages show no correlation. In other words, if the reticle stage and the wafer stage both show a substantially same error, its effect may be significantly less than an effect of uncorrelated errors of the stages.
To obtain a high tracking, i.e., to make sure that movement and/or a position of the reticle stage and wafer stage match as good as possible, thus showing a low relative error, a feedforward may be included which provides a feedforward signal to e.g., the reticle stage based on a setpoint input provided to the wafer stage or based on an error signal in a control loop controlling a position of the wafer stage. A problem however with such a feedforward is that errors still remain present due to a finite bandwidth of the stages and of the control loops.